A surface acoustic wave (SAW) element is extensively used as a band pass filter for communication equipment such as a cellular phone. With the enhancement in performance of the cellular phone or the like, higher performance is required also in such a filter using the surface acoustic wave element.
However, the surface acoustic wave element has a problem in that its passband is shifted with temperature changes. Lithium niobate or lithium tantalate presently in heavy usage is particularly advantageous for achieving wideband filter characteristics due to its large electromechanical coupling coefficient. However, lithium niobate or lithium tantalate is inferior in temperature stability.
For example, the temperature coefficient of frequency change of a surface acoustic wave filter using lithium tantalate is −35 ppm/° C., and frequency variation in a possible operating temperature range is large. Therefore, the temperature coefficient of frequency change is required to be reduced.
In a surface acoustic wave element described in Patent Literature 1 (Japanese Patent Publication No. H05-335879A), the temperature coefficient of frequency is reduced by forming, after forming an interdigitated electrode on a surface of a lithium niobate substrate, a silicon oxide film so as to cover the substrate surface and the interdigitated electrode.
In a surface acoustic wave element described in Patent Literature 2 (Japanese Patent Publication No. 2009-278610A), the temperature coefficient of frequency change is successfully reduced, by adhering a support substrate, composed of silicon or the like having a smaller thermal expansion coefficient, to a propagation substrate composed of lithium tantalate single crystal or the like, through an organic adhesive layer having a thickness of 0.1 to 1.0 μm.